This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. All living organisms have an ability to respond to biological stimuli such as light, voltage, oxygen, ligands and environmental factors. When applied, each stimulus is converted into a specific cellular response in several steps contributing to signal-transduction pathway. To understand the working mechanism of signal transduction, it is important to identify the molecular units involved in signaling, for example, stimuli receptors. Among the stimuli, light plays an important role in a wide range of biologically important cellular responses including photosynthesis and gene expression (1). Thus far, six kinds of photoreceptors have been identified (2). Among these photoreceptors, the BLUF domain was discovered recently, thus having been studied to a limited extent. In particular, the Slr1694 BLUF domain exists as a homo dimer in solution, but oligomerizes in the dark to form a decamer. This putative signaling state is optically inactive that its formation dynamics is not readily probed using time-resolved optical spectroscopy, for example, transient absorption (TA) spectroscopy. In contrast, time-resolved X-ray scattering techniques are sensitive to the structural changes regardless of the optical activity of the state of interest. For example, time-resolved small-angle X-ray scattering (TR-SAXS) is sensitive to global structural changes such as size and shape of a protein in solution, while time-resolved wide-angle X-ray scattering (TR-WAXS) provides rich information about tertiary and quaternary structural changes of a protein with global sensitivity. Therefore, time-resolved X-ray scattering techniques are suitable for probing the structural dynamics of the signaling state formation in the BLUF domain proteins. Here, we propose the time-resolved experiments on the Slr1694 BLUF domain proteins using TR-SAXS and TR-WAXS techniques to investigate the mechanism of photochemical reaction leading to the formation of signaling state.